Undercut mesa geometries are used as semiconductor heterostructure waveguides, buried heterostructure light sources or exposed mesa light sources. Undercut mesa geometry refers to the fact that, in a heterostructure, a lower layer of the heterostructure is partially etched along at least one exposed edge so that an upper layer is wider than the lower layer thereby forming a mesa-like cross-sectional shape in a plane cutting through all heterostructure layers. In other words, upper layers of the multilayer heterostructure overhand the undercut lower layer. There may be additional layers below the undercut lower layer. The lower layer is generally a light guiding or active layer of the heterostructure whereas the layers surrounding the lower layer are cladding layers.
Undercut mesa geometries have been reported in which the edges of the undercut, guiding layer of the heterostructure are directly exposed to either a vacuum or air. See, for example, U.S. Pat. Nos. 3,833,435 and 3,883,219 issued to R. Logan et al., an article by H. Burkhard et al. in Japanese Journal of Applied Physics, Vol. 22, No. 11, pp. L721-3 (1983), and an article by R. Blondeau et al. in Japanese Journal of Applied Physics, Vol. 21, No. 11, p. 1655 (1982). As a result of direct exposure to vacuum or air, the guiding layer forms a two-dimensional waveguide exhibiting very strong light guiding properties. Strong guiding results from the significant refractive index difference at the exposed edges of the guiding layer. As such, it is inherently difficult to achieve fundamental transverse mode operation unless the waveguide width is very small. In practice, the transverse mode is controlled by the high scattering loss from imperfections at the large index discontinuity at the waveguide edges.
For some lasers incorporating a similar undercut mesa geometry, the undercut regions are filled with silicon dioxide and polyimide polymer films. See U. Koren et al., Applied Physics Letters, Vol. 42, No. 5, pp. 403-5 (1983). The combination of silicon dioxide and polyimide films still results in strong index guiding and high scattering loss. Again, this scattering loss is used to control transverse mode behavior.
In yet another laser structure incorporating the undercut mesa geometry, a mass transport process is employed to fill in the undercut regions with semiconductor material. See Z. Liau et al. Applied Physics Letters, Vol. 40, No. 7, pp. 568-70 (1982); T. Chen et al., IEEE Journal of Quantum Electronics, Vol. QE-19, No. 5, pp. 783-5 (1983); T. Chen et al., Journal of Applied Physics, Vol. 54, No. 5, pp. 2407-12 (1983); and A. Hasson et al., Applied Physics Letters, Vol. 43, No. 5, pp. 403-5 (1983). The semiconductor material which fills in the undercut regions migrates from other portions of the structure as a result of high temperature transport phenomena. As such, the composition and conductivity of the semiconductor material which fills in the undercut regions are not under independent control but rather are determined by the locally available exposed semiconductor material such as the material comprising upper (overhanging) and lower cladding layers of the heterostructure. Since the mass transported material has a small refractive index mismatch with respect to the undercut layer, the resulting waveguide is capable of optical confinement without encountering large scattering losses due to the presence of imperfections. Additionally, wider waveguides may be fabricated while maintaining fundamental transverse mode operation.
However, in the case of an active (pumped) waveguide, operation of the resulting waveguide ca be substantially degraded by DC leakage currents and high frequency displacement currents because the original highly doped semiconductor material from the cladding layers is in the conduction path including the lower cladding layer, the filled-in undercut regions, and the upper layers of the undercut mesa. Because of these leakage currents, an active device, such as a laser, incorporating this waveguide structure has a degraded high frequency response.
Furthermore, with respect to the mass transport process, it has been found that complete filling of the undercut regions is dependent upon the thickness of each undercut region which, in turn, directly depends upon the thickness of the undercut lower layer. For a relatively thin undercut layer of approximately 0.2 .mu.m, mass transport appears to perform adequately for filling in the undercut regions adjacent to the undercut layer. However, for a thicker undercut layer, for example, a thickness of approximately 0.3 .mu.m or greater, which is often desirable in heterostructure devices guiding light at longer wavelengths, such as .lambda.=1.5 .mu.m, mass transport is less effective for filling the undercut regions. It appears, for example, to cause irregularities in the regrown material in the undercut regions leading to a waveguide having poor optical properties or to transport insufficient material. Additional drawbacks of the mass transport method reside in the relatively high temperature and long time required. It will be readily appreciated that the original dopant distribution may be changed by diffusion during the mass transport process.